Synthetic membranes are being utilized in kinetics studies to provide a means for continuous sampling of the liquid phases from systems in which a dispersed particulate phase is suspended in the liquid phase. In one application sampling equipment is being developed for in vitro study of calcium ion transport and calcium-ATPase activity in suspensions of sarcoplasmic reticulum vesicles prepared from homogenates of rabbit muscles. In a second application a study of the mammalian blood-brain-barrier permeability is being aided by the development of an apparatus incorporating a sampler in an arteriovenous ex vivo shunt. In the latter plasmapheresis application, pooling of the plasma filtrate yields a single sample from which the plasma concentration-times-time integral can be evaluated for a chemical adminstered to the animal. Such sampling systems can be useful for the study of the kinetics of other fluid phase systems for which a membrane can be found that is permeable to one chemical of interest but impermeable to another necessary reagent or sink. Thus, other applications might be found in the areas of enzyme kinetics, pharmacokinetics, and the membrane transport of vesicle and cell suspensions.